The present invention relates to apparatus for starting and for operating, on direct current, gaseous discharge lamps such as fluorescent lamps from a source of alternating current.
Fluorescent lamps are widely used for domestic and industrial lighting because they generate less heat and are more efficient than other common light sources such as incandescent lamps. They are ordinarily operated on alternating current (AC) because AC power sources are readily available and because AC systems provide certain advantages in ballasting and obtaining adequate starting voltage for the lamp. However, AC operation of fluorescent lamps has several disadvantages. Because the arc in the lamp must strike at twice the frequency of the AC supply current a flickering light is produced. Thus, when operated on 60 H.sub.z current, the lamp produces 120 flashes per second. This flickering effect can cause headache and eyestrain in some individuals and can cause epileptics to go into seizure. Also, because of the rapid rise in arc current in the lamp at each flash, the lamp emits a broad band of radio frequency radiation which causes interference problems in lighting applications where there is sensitive electronic equipment in an area lighted by fluorescent tubes.
To obviate the above difficulties of light flicker and unwanted radiation gaseous discharge lamps can be operated on direct current and this has been done where direct current (DC) power sources are conveniently available as on some subway trains. However, the available power supply is ordinarily AC and this requires the use of special rectifier, ballast and starting circuits for the lamp which have heretofore been expensive and relatively inefficient. This has limited the use of DC operated fluorescent lamps to a few special cases where DC power supplies are available or special applications such as photoprinting lights where the extra cost of the power rectification apparatus can be justified.
The reason prior apparatus for operating gaseous discharge lamps on DC from an AC source has been expensive and relatively inefficient stems from the electrical operating characteristics of such lamps. It is well known that they have an inherent negative resistance-current characteristic so that after the lamp arc strikes, the lamp current will become excessive unless a suitable ballasting device is used. Ballasts used in DC systems heretofore have caused substantial power loss often exceeding the loss in the lamp itself. Also, such lamps normally have a starting voltage substantially higher than the operating voltage. In DC systems the rectifiers must be able to withstand the high starting voltage or alternatively some automatic control system must be provided to isolate the rectifiers from the high starting voltage. Also, in such systems the starting voltage has been obtained by using apparatus such as a pulse transformer for superimposing AC on the DC current supplied to the lamp electrodes. This AC will cause the lamp to flicker unless manual or automatic apparatus is provided to remove it from the circuit after starting. Such apparatus adds considerably to the cost.
Accordingly, it is an object of the invention to provide improved apparatus for operating gaseous discharge lamps on DC from a source of AC that is less costly and more efficient than apparatus heretofore used for this purpose.
Another object of the invention is to provide an improved circuit and apparatus for operating gaseous discharge lamps without flicker and without objectionable radiation in the radio frequency range.
A still further object of the invention is to provide apparatus for starting and operating gaseous discharge lamps that does not require a ballast transformer or series reactor to obtain stable operation.
Further objects and advantages of the invention will become apparent as the following description proceeds.